The use of polymer materials in the dental arts for the restoration of lost or damaged teeth is well known. Such uses fall into two general categories: (i) the use of polymer materials to produce dental prosthetics, such as dentures, bridges and crowns, that are either permanent or removable articles, and (ii) the use of polymer materials to create dental composites for fillings to repair teeth instead of using conventional amalgam fillings or as veneers to refinish tooth enamel surfaces. The first category of dental articles, dental prosthetics, are created outside of the patient (i.e., extra-oral), typically by making an impression of what the desired article should look like and then molding the article to match the impression. The second category of dental articles, dental composites, are created directly in the patient's mouth (i.e., intra-oral) as fillings or veneers to repair or resurface teeth.
The use of microwave energy in the form of a commercial microwave oven used to process dental prosthetics extra-orally is well known. Various types of flasks and molding equipment that can be used in conjunction with a commercial microwave oven for processing and curing dental articles made of polymers have been developed as described, for example, in U.S. Pat. Nos. 4,971,735, 5,151,279, 5,324,186 and 5,510,411, European Patent No. 0 687 451 A2 and Japanese Patent No. JP7031632A. Examples of polymer resin matrices that are specifically formulated to utilize microwave energy supplied by a commercial microwave oven for the thermal polymerization of the polymers into dental articles are shown in U.S. Pat. Nos. 4,873,269, and 5,218,070 and Canadian Patent No. 2,148,436. The impact of the role played by the polymer initiator in a microwave cured resin matrix has been evaluated by Urabe H. et al. in “Influence of polymerization initiator for base monomer on microwave curing of composite resin inlays,” Journal of Oral Rehabilitation, Vol. 26, pp. 442–46 (1999). The repair of dentures and related articles using microwave processing is also described in Turck M D et al, “Microwave processing for dentures, relines, repairs and rebases,” The Journal of Prosthetic Dentistry, Vol. 69, No. 3, pp. 340–43 (1993). Generally, dentures cured by commercial microwave ovens have improved mechanical properties, and often have better adaptation than those cured by conventional water-bath method. The primary advantage of microwave curing, however, is the reduced processing times which can be shortened from 8 hours or more to as little as a few minutes.
There has been relatively little research, however, into the potential impact of the microwave energy itself on the polymerization process for dental prosthetics. The research that has been done has generally focused on the duty cycle used for the microwave oven curing process. The impact on porosity of denture material cured using lower wattage, longer duration microwave cure times (i.e., a lower duty cycle for a longer time) versus higher wattage, shorter duration microwave cure times (i.e., a higher duty cycle for a shorter time) is compared in Alkhatib M B, et al. “Comparison of microwave-polymerized denture base resins,” The International Journal of Prothodontics, Vol. 3, No. 2, pp. 249–55 (1990). European Patent No. 0 193 514 B1 describes a microwave processing system for dental prosthetics that has a magnetron, a waveguide, a surface radiating antenna, a flask, and a temperature sensor that is inserted in the flask and connected to a regulating processor. The regulating processor limits the temperature in the flask as measured by the temperature sensor by turning on and off the magnetron based on frequency modulation of the duty cycle. Although not used for polymerization of dental articles, U.S. Pat. No. 5,645,748 does describe a microwave system for sterilization that controls duty cycle of a microwave oven for the purpose of minimizing arcing caused by metallic surgical or dental instruments.
With respect to the second category of dental articles created using polymer materials, dental composites formed of polymer matrix-composites are increasingly being used as an alternative to mercury-containing dental amalgam for aesthetic and restorative dental materials. These kinds of polymer matrix-composites are usually photo polymerizable in that they are cured using some kind of light instead of heat. Generally, the polymer matrix-composite is based on a photo polymerizable polyfunctional methacrylate compound that can be used alone or as a mixture with monomethacrylates, light sensitive cure initiators pigments and fillers in a mixture with various comonomers such as triethyleneglycol dimethacrylate. Although the half-life of these polymer matrix-composites cured by light is on the order of 5–8 years and therefore they tend to wear out earlier than conventional dental amalgams, the enhanced biofunctionalilty and more pleasing aesthetic qualities of these polymer matrix-composites have gained favor over conventional dental amalgams.
The main deficiencies of polymer composite resins used as dental composites are surface degradation which leads to inadequate wear resistance, polymerization shrinkage and a lack of density. In addition to the problems previously described for dental prosthetics, micro-shrinkage of polymer dental composites produces interfacial gaps on the surface of the composites, which can result in microleakage through the dental composite. The long-term consequence of such microleakage can be bacterial penetration into the tooth that can cause a variety of adverse reactions such as pulp damage, tooth sensitivity, possible pulpal death and loss of adhesion of the dental composite.
Improving the degree of polymerization of polymer matrix-composites is generally considered to be one way of improving their physical and biofunctionality characteristics of polymer dental composites as this would lead to stronger dental composites that are less susceptible to degradation, wear and fracture. It would also lead to improved biocompatibility, since there would be reduced amounts of uncured monomer that could act as a biohazard.
Unlike polymer dental prosthetics, however, the curing of polymer matrix-composites by application of thermal energy generally has not been used to date. Obviously, in the case of the conventional thermal water-bath process, it would be impractical to require a patient to remain at the dentist's office for up to 8 hours with their mouth open and with a tooth immersed in a hot water bath in order to set a thermally polymerizable matrix-composite. It is also not possible to place a patient's mouth into a commercial microwave oven to set a thermally polymerizable matrix-composite.
While there are numerous hand-held medical catheter devices that utilize radio frequency and microwave energy to perform ablations and similar heating operations, for example, in the vascular system of a patient, there have been relatively few uses of thermal or electrical energy applied to hand-held dental tools for intra-oral applications. There have been a few hand-held dental probes that utilize an electrically resistive heated tip for diagnosis of dental decay or for melting a sealing material in an intra-oral context as described, for example, in U.S. Pat. Nos. 4,527,560 and 5,893,713. U.S. Pat. No. 5,421,727 describes the use of radio frequency/microwave energy as part of a hand-held endodontic root canal device to raise the temperature of the interior of the tooth adjacent to the root canal, thereby tending to disinfect the tooth during the root canal procedure as a result of the increased temperature.
The extra-oral use of microwave energy for the purpose of characterizing dental decay in extracted teeth has been described by N. Hoshi et al in “Application of Microwaves and Millimeter Waves for the Characterization of Teeth for Dental Diagnosis and Treatment,” IEEE Transactions on Microwave Theory and Techniques, Jun. 1998, Vol. 46, No. 6, pp. 834–38. This study confirmed the higher absorbency behavior of carious lesions in extracted teeth when irradiated by microwave energy as compared to the lower absorbency of such microwave energy by healthy enamel and dentin.
While existing photo polymerizable dental composites have enjoyed success as compared to conventional dental amalgams for dental fillings and veneers, it would be desirable to further improve the uniformity and degree of conversion of monomers into polymer chains in the polymerization process in order to produce even better dental composites. It would also be desirable to provide a dental tool that could take advantage of the use of microwave energy for purposes other than the polymerization of dental composites.